The present invention generally relates to lighting systems and related technologies. More particularly, this invention relates to fluorescent lamps and coating systems utilized by fluorescent lamps to generate visible light.
Fluorescent lamps have been in use and commercialization since the 1930s. More recently, both consumers and producers have voiced increased concerns for energy efficiency and environmental impact of products, spanning all industries, including the lighting industry. As such, fluorescent lights have seen an increase in usage due to their increased energy efficiency when compared to conventional incandescent lights. Fluorescent lights see a great deal of competition from light-emitting diode (LED) lights, due to a potential for greater efficiency and luminosity of LEDs. Significant effort and research have been made in the interest of improving fluorescent light lumen output without increasing power requirements or significantly increasing material costs.
A nonlimiting example of a fluorescent lamp 10 is schematically represented in FIG. 1. The lamp 10 is represented as having a sealed glass tube comprising of a glass envelope or shell 12 enclosing an interior chamber 14. The chamber 14 is preferably at very low pressure, for example, around 0.3% atmospheric pressure, and contains a gas mixture having at least one constituent that can be ionized to generate radiation that includes ultraviolet (UV) wavelengths. According to the current state of the art, such a gas mixture includes one or more inert gases (for example, argon) or a mixture of one or more inert gases and other gases at a low pressure, along with a small quantity of mercury vapor. Electrodes 16 inside the chamber 14 are electrically connected to electrical contact pins 18 that extend from oppositely-disposed bases 20 of the lamp 10. When the contact pins 18 are connected to a power source, the applied voltage causes current to flow through the electrodes 16 and electrons to migrate from one electrode 16 to the other electrode 16 at the other end of the chamber 14. In the process, this energy converts a small amount of the liquid mercury from the liquid state to a charged (ionized) gaseous (vapor) state. The electrons and charged gas molecules move through the chamber 14, occasionally colliding with and exciting the gaseous mercury molecules, raising the energy level of the electrons in the mercury atoms. In order to return to their original energy level, the electrons release photons.
Due to the arrangement of electrons in mercury atoms, most of the photons released by these electrons are in the ultraviolet (UV) wavelengths. This is not visible light, and as such for the lamp 10 to emit visible light these photons must be converted to a visible light wavelength. Such a conversion can be performed by a coating 22 represented in FIG. 1 as disposed at the interior surface of the glass shell 12. The coating 22 contains phosphor powders and is often separated from the glass shell 12 by a UV-reflecting barrier layer 24 of, for example, alumina (Al2O3). The UV radiation emitted by the ionized mercury vapor is absorbed by the phosphor composition within the coating 22, resulting in excitation of the phosphor composition to produce visible light that is emitted through the glass shell 12. More particularly, when electrons of the phosphor atoms are struck by photons, the electrons become excited to a higher energy level and emit a photon to return to their original energy level. The emitted photon has less energy than the impinging photon and is in the visible light spectrum to provide the lighting function of the lamp 10. The color and luminosity of the lamp 10 are largely the result of the phosphor or phosphors used in the coating 22.
The apparent, or perceived, color of a light source can be described in terms of color temperature, which is the temperature of a black body that emits radiation of about the same chromaticity as the radiation considered. A light source having a color temperature of 3000K has a larger red component than a light source having a color temperature of 4100K. As additional examples, a fluorescent lamp having a perceived “warm white” color may have a CCT of approximately 3000K, whereas a fluorescent lamp having a perceived “cool white” color may have a CCT of approximately 4000K. Another measure of fluorescent lamp performance is the color rendering index (CRI). The CRI of a light source does not indicate the apparent color of the light source, but instead is a quantitative measure of the ability of a light source to reproduce the colors of objects faithfully in comparison with an ideal or natural light source. CRIs can only be accurately compared among two light sources having the same CCT. The highest possible numeric CRI value is 100. Incandescent lamps, which are essentially blackbodies, have CRIs of 100. Typical LEDs have CRIs of 80 or more, with CRIs of up to 98 being claimed, whereas fluorescent lamps typically have CRIs in a range of about 50 to about 90. In this regard, a high CRI for fluorescent lamps can be considered to be about 80 and higher, particularly at least 87.
Because fluorescent lamps utilize phosphors to produce visible light, the CRI and CCT of a fluorescent lamp is strongly influenced by the particular amounts of phosphors used and their compositions. Significant improvements in the CRIs of fluorescent lamps have been achieved with rare earth phosphors, in particular, phosphors containing one or more rare earth elements generally considered to include the fifteen lanthanides, scandium, and yttrium. Though fairly abundant, rare earth elements are becoming increasingly prohibitive to procure due to the majority of known rare earth reserves being found in limited locations, and the need to separate individual rare earth elements or compounds from relatively low concentrations found in various mined compounds. In addition, demand for rare earth elements has increased in a wide variety of technologies, for example, medical technologies, wind turbines, hybrid automobiles, televisions, smart phones, and computers. As such, the use of rare earth elements in phosphor coatings of fluorescent lamps has or may become increasingly cost-prohibitive, and effective alternative solutions are being sought. However, suitable alternatives preferably should not detrimentally affect the CRI or CCT of a fluorescent lamp.